George WeigelBlack plastic under a stone path is a better idea than in garden beds.Q: I’ve been trying to stop weeds by laying black plastic underneath my mulch. Every year or two, I pull it up and till, then put it back down. Is there any problem with doing that? It seems to help with the weeds, but it’s a lot of work

A: That’s for sure! I’m not a big fan of black plastic, except in the vegetable garden with a soaker hose or drip line underneath and in pathways underneath stone.

The main downside is that plastic reduces oxygen exchange in the soil. Roots need oxygen, so whether it’s excess water filling the air spaces or a plastic barrier shutting off the flow, less oxygen is bad news for plant growth.

Years and years ago in my “garden infancy,” I laid plastic down and planted a bed of groundcover junipers. They didn’t seem to be doing very well after a few years. When I uncovered an area, I found that the juniper roots had grown up and sucked onto the bottom of the plastic – almost as if they were gasping for air and crying, “Let me out! Let me out!”

Plastic is especially bad in clay soil, which doesn’t drain well to start with. The combo of reduced evaporation from excess moisture and reduced oxygen exchange is a plastic double whammy that can stunt or kill plants.

In dry weather, plastic doesn’t help either because when rain comes, it can only seep in around the openings where you’ve inserted plants.

Another problem… when mulch starts to break down over top of plastic mulch, weeds can germinate on top. That starts to happen the second or third year and gets progressively worse unless you keep adding more and more mulch. And if you’re going to do that, why bother with the plastic in the first place?

You’re short-circuiting this by removing the plastic and topping it with fresh mulch every year or two. That’ll do a great job at stopping weeds, but like you say, it’s a ton of work.

I’m not sure what benefit tilling would give you, other than possibly aiding aeration so long asyou’re not tilling when the soil is wet. On the other hand, tilling will stir up buried weed seeds, disturb earthworm activity (you may even chop some of them up in the process), injure nearby plant roots and possibly harm the soil structure.

I think the best anti-weed strategy is 1.) plant so the plants you want touch, 2.) plant groundcovers under and between woody plants, 3.) keep 2 or 3 inches of bark mulch and/or chopped leaves over bare soil, and 4.) pull weeds that manage to pop up anyway when they’re young.